Between the hours of 6--10 UT on May 24, 2000, the IMAGE extreme ultraviolet (EUV) instrument observed a shoulder-shaped bulge in the morning sector plasmapause <Burch et al., 2001a, 2001b>. Simulation results of the data-driven Magnetospheric Specification Model (MSM) have reproduced the formation (during 4:00--5:15 UT) and subsequent evolution of the shoulder. In the model, the shoulder is created by a dusk-to-dawn overshielding electric field, triggered by two sudden, strong northward (Nwd) turnings of the IMF. Overshielding causes antisunward flow of pre-dawn plasma, producing an asymmetric bulge that rotates eastward.